1. Field of the Invention
This invention relates to a video display system of the kind in which at least one visible characteristic of consecutive image points on the screen of a raster-scan CRT is defined by the values of consecutive pels of a digital video drive waveform, each such pel comprising one or a plurality of video bits in parallel, and in which a pulse stretching circuit is provided for extending the duration of selected pels in the video waveform in order at least partially to compensate for image distortion introduced by the finite video amplifier rise and fall times of the CRT. One system of this kind is described in IBM TDB, Vol. 24, No. 11B, page 5794 and is used in the IBM 8775 terminal, another is described and claimed in the European patent application No. 0104289.
2. Background Art
The video channel of a high content raster-scan CRT display must operate at a very fast data rate if flicker is to be avoided. For example, a data display having 1.2 million image points refreshed at 60 Hz with a non-interlaced raster requires a peak data rate of about 100 Mpels/Sec. This corresponds to a pel period of 10 nSecs. Full modulation of the electron beam requires a cathode drive voltage of about 35 volts for a monochrome tube and up to 60 Volts for color. It is very difficult to design a video amplifier to product these voltage transitions in a time which is short compared to the pel period. This is particularly true if the amplifier must handle analogue signals rather than a simple binary waveform. In this case 10 to 90% rise and fall times of 7 nSecs are considered state-of-the-art for a color display. Such an amplifier will product greatly distorted video pulses compared to the ideal rectangular shape. For the user the effect is particularly noticeable on vertical strokes which have much reduced contrast if they are only one image point wide. The problem is most severe with a monochrome bright-on-dark display (hereinafter referred to as a white-on-black display for convenience) because the beam current is proportional to the drive voltage raised to a power gamma, where gamma is typically 2.2. consequently, the contrast of a single image point is effectively related to the drive pulse width measured near the voltage for peak white and this is only a few nSecs for a white pulse with the figures quoted.
One known solution to this problem, used in white-on-black displays and described in the above referenced IBM TDB is to extend the trailing edges of positive (white) pels by logically OR'ing the video waveform with a delayed version of itself. Obviously this technique lengthens positive pels by shortening negative ones and as such as unsuitable for displays having mixed white-on-black and black-on-white information. This problem can be overcome in the restricted case when all the information in a particular region of the display screen is known by the system to have the same polarity. In this case the video signal can be inverted before and after the basic pulse stretching circuit by two exclusive OR gates fed with a signal indicating the information polarity.
In order to cope with mixed polarity displays of high density the system itself must have knowledge of the polarity of the display in each region of the screen. Even where the polarity is known, a highly dense display would have a significant number of image points of opposite polarity to the main information which are isolated in the raster scan direction, and such points would inevitably be reduced in width by the automatic delay of the trailing edge of the immediately preceding pel. Also, the technique cannot be extended to displays with several bits per pel.
An improved display system of the above kind which overcomes these drawbacks is described in an aforementioned European patent application. Here the pulse stretching circuit comprises decoding means for examining each pel at least in relation to its two immediate neighbors on either side in order to detect predetermined relationships between the values of the pels, and retiming means for selectively advancing or delaying the transitions between consecutive pels of different value in accordance with the relationships so detected. The advantage of this system over that described in the TDB article is that pels are selected for extension only as a function of their relationship to neighboring pels, so that isolated pels of substantially different color and/or intensity to their neighbors can be identified, at least maintained at their nominal width, and where possible increased in width.
Although the system described in the aforesaid European patent application provides substantially improved visual results for highly dense or mixed video pictures, and considerably enhances the front-of-screen performance of the display system compared to the existing technique, the penalty for this improved performance is one of cost in the additional circuitry involved. Thus, even in the simple case where only the relationship of each pel to its two immediate neighbors on either side is examined, the preferred circuitry includes a 5 stage shift register; a multi-bit comparator; a 3-stage shift register with associated output logic; and 3 clocking latches also with associated output logic. As explained, the system is not restricted to this simple case and by providing more complex circuits at further cost it is possible to compensate for image distortions in both color and monochrome to an increased degree of sophistication.
As previously stated hereinbefore, it has been observed that the problem of image distortion is most severe in the case of a white-on-black display. Thus, whereas a single pel wide vertical white line on a black background virtually disappears, a single pel wide vertical black line on a white background is clearly visible. As a result of an analysis of the reasons for the distortion, a new solution to the problem has been devised which not only has the advantage over the previous solutions of being applicable to bi-level (black and white) high resolution displays and to an image display with a number of `grey` levels in monochrome or in color but also is of relatively low cost. In addition to the saving of circuitry, the present solution is believed to provide a more accurate correction than that afforded by previous methods.
The analysis of the problem will now be given with reference to FIGS. 1 to 3 of the accompanying drawings. FIG. 1 shows the relationship between the grid (or cathode) voltage V.sub.d and beam current I.sub.b in a cathode ray tube display. Beam current is linearly related to the resultant luminance of the screen phosphors. The relationship between the applied grid voltage V.sub.d and beam current I.sub.b is given by the expression: EQU I.sub.b =.kappa..Vu.sub.gamma,
where .kappa. and gamma are constants
In FIG. 1 two relationships are shown, continuous curve 1 for the case where the gamma value is 1 and broken curve 2 where the gamma value is 3. In practice, in order to compensate for the non-linearity of the response of the human eye, a CRT with a gamma value of 2.2 is probably ideal.
Two curves illustrating the relationship of grid voltage to beam current of the video amplifier are shown in FIG. 2 in response to the rising and falling edges of a single bit of a video signal bi-level waveform 3 such as might be applied as input to a CRT grid electrode. An input step function V.sub.in applied to a CRT grid electrode is shown as waveform 3. The resulting grid voltage V.sub.d is shown as continuous waveform 4. It might be expected that the resultant beam current would follow the grid voltage faithfully. The continuous curve 4 represents the theoretical response of the amplifier. However, in view of the non-linear relationship between grid voltage and beam current in practice, with a CRT gamma of 3, the beam current I.sub.b and hence the light emission is more likely to be as represented by the broken curve 5. The waveforms 3, 4 and 5 are all shown on the same time scale in FIG. 2. It is seen therefore that the rise time of an input pulse is made effectively longer while the fall time is made shorter. Furthermore, the amplitude of the effective video amplifier pulse never reaches its intended full magnitude represented by the magnitude of the bi-level input signal waveform 3 and the displayed pel on the screen never reaches full brightness.
In FIG. 3 two portions of a bi-level input waveform leading to opposite conditions on the screen are shown. The first, waveform portion 6, shows the signal applied to a video amplifier in order to display a single white or `on-pel` on each side of which is a black or `off-pel`. The second, waveform portion 7, shows the signal required in order to display the condition where a black or `off-pel` is sandwiched between two white or `on-pels`. Superimposed on these two input waveform portions in FIG. 3 are the effective resulting video amplifier output waveforms shown as continuous curves 8 and 9 respectively, which actually drive the video guns of the CRT.
Thus in the case where the portion 6 of the input pulse waveform calls for the display of a single white pel sandwiched between black pels, the resultant video output pulse 8 is distorted as described hereinbefore with reference to FIG. 2. that is, it is shorter in duration than the input waveform that drives it and does not reach its intended full magnitude. Accordingly, the resulting white pel is narrower and of lower intensity than desired.
In the case where the input pulse waveform 7 calls for the display of a single black pel sandwiched between white pels, the response to the rising and falling edges of the input pulse by the video amplifier is just the same, but because of the inverted input pulse condition, produces a different effect. As shown in the figure, the video output pulse falls fairly rapidly in response to the trailing edge of the input pulse but is relatively slow to respond to the subsequent rising edge. The effect of this is to cause the video output to be at its down level for a longer period than that called for by the input pulse. Consequently, the width of the resulting black pel is wider than required. This increase of width of the black pel is not so noticeable to a viewer of the CRT screen as the reduction in width and intensity of a white pel.
With multi-grey level waveforms the effect of the non-linearity on the amount of emitted light is substantial. The differences between shades are compressed in the darker grey shades and expanded in the lighter grey shades. It is therefore highly desirable not only to mitigate the delaying effect on white edges but also on all transitions from darker to lighter shades of grey. There is no need to provide any correction for black edges or for transitions from lighter to darker shades of grey.
The above analysis of the problem has shown that the effect of gamma on the fall time of the video amplifier output is beneficial in the way that the resulting waveform resembles the ideal rectangular edge closer than that of the raw output from the video amplifier. As a consequence, the CRT beam current and thereby the CRT luminance decrease quicker. On the other hand, the effect of the slowed down rise-time is severe. The additional delay in the typical case for a gamma of 3.5 and pulse video amplifier rise time of 2 ns is about 1.5 ns and is the same for all steps in the video signal representing dark to light transitions on the screen.